Regulation of the cell cycle

MPF (internal control)

(MITOSIS PROMOTING FACTOR) MPF is a cyclin-CDK complex which acts at the G2 checkpoint which triggers the cells undergo mitosis. •(MPF activity corresponds to high levels of cyclin it is active when cyclin concentration is high. •MPF is considered an internal signal because it is produced within the cell.

Cell Cycle Control System

A cyclically operating set of molecules that trigger and cordinate events in the cell cycle

CDKS

To be active, kinases must be attached to a cyclin protein. Termed CYCLIN DEPENDENT KINASES. The activity of CDKS rises and falls with the changes in the concentration of its cyclin partner

Growth factors (external control)

• a protein released by certain cells that cause other cells to divide

G1 checkpoint

•If cell receives the go ahead signal it usually completes the entire cell cycle and divides

How does MPF trigger mitosis?

•Phosphorylates proteins of the nuclear me,brand triggering its breakdown •activates other kinases, which the go on to Phosphorylate molecules resulting in their activation •involved in chromosome condensing •involved in mitosis spindle formation

PDGF (External factor)

•Platelets produce a growth factor called PDGF. •When damage to a blood vessel occurs, platelets release PDGF. •PDGF acts on a type of tissue called fibroblasts •PDGF attaches to receptors on the plasma membrane of these cells and stimulated cell division •fibroblasts are needed to help seal the wound •different cells respond to different growth factors

Kinases

•Special enzymes that activate or inactivate other proteins by phosphorylation get them. (Adding a phosphate group). •Specific kinases give the go ahead signals at g1 & g2 Checkpoints •most of the time in inactive form •many kinases involved in regulating the cell cycle are present in constant concentration in a growing cell

Anchorage dependence

•animal cells do not divide unless attached to a substratum

Density dependent inhibition

•cells stop dividing if there is a limited space Ex) single layer of cells on a Petri dish (the binding of a cell surface protein to the neighboring cell inhibits further division)

How does MPF end mitosis?

•during anaphase, the cyclin part of MPF is degraded ending mitosis,

G0 phase

•if cell does not receive go ahead signal, it enters a non-dividing stage called G0 Ex) muscle and nerve cells don't divide But liver cells respond to growth factors and can be called back, from the G0 phase. Moving from G0 to G1

M phase checkpoint (internal control)

•in anaphase,, the separation of sister chromatids does not begin until all the chromosomes are properly attached to the spindle at the metaphase plate •kinetochores not yet attached to spindle microtubules send a molecular signal that prevents separation of the sister chromatids. •which ensures that daughter cells do not end up with missing or extra chromosomes.

Cyclins

•regulatory proteins that activate kinases •cyclin concentration in a cell fluctuates cyclically

Checkpoints

•triggered by internal and external controls at certain points •control point where molecules can regulate the the cell cycle